In response to injury, cancer, microbial invasion, and the like, humans mount inflammatory reactions to control the pathological condition and to initiate a repair process. During inflammation, various immune cells including T-lymphocytes, neutrophils and macrophages are recruited to the site of infection and produce cytokines to facilitate the immune response. Among these cytokines, tumor necrosis factor-α (TNF-α) is one of the major proinflammatory proteins to mediate the immune defense.
In addition to acute phase response, TNF-α has been shown to be involved in the progression of various chronic diseases including tumorigenesis and rheumatoid arthritis (RA). The dysregulation of TNF-α production was demonstrated to be involved in different stages of tumorigenesis including initiation of tumor growth1, cell proliferation2 and invasion3. For tumor cell proliferation, TNF-α upregulates specific growth factors to mediate the malignant growth. The cytokine promotes angiogenesis favoring growth of blood vessels to support the tumor migration, and thus plays a key role in tumor metastasis. For example, glioblastoma migration and induction of metalloproteinases are significantly enhanced in response to TNF-α effects4.
Examples of chronic disease pathogenesis mediated by TNF-α include rheumatoid arthritis and inflammatory bowel diseases. Patients with rheumatoid arthritis have a low grade insidious inflammation in the synovial tissues. It is known that overproduction of TNF-α at the inflamed joint leads to slow destruction of the joint cartilage and surrounding bone.
During an acute phase of infection such as in the case of sepsis, uncontrolled production of TNF-α is well known to cause deleterious effects to the host. Sepsis is the second most common cause of death in non-coronary intensive care units and the tenth leading cause of death overall in high-income countries5. The clinical outcome of infection leading to sepsis is primarily associated with the excessive stimulation of the host immune cells, particularly monocytes or macrophages, by bacterial endotoxins (e.g., lipopolysaccharide [LPS])6-8. Macrophages overstimulated by LPS also produce high levels of mediators such as interleukin-1 (IL-1), IL-6, and TNF-α9. These mediators are implicated in the pathogenesis of sepsis and found to be contributing factors to the demise of the host. The development of novel therapies directed towards the inhibition of TNF-α production may help to aid in the treatment of these acute and chronic diseases described above.
Following exposure to pathogens and endotoxins, intracellular signaling pathways including specific kinases and transcription factors are activated to induce the expression of TNF-α. The involvement of mitogen-activated protein (MAP) kinases and the nuclear factor kappa B (NF-κB) in pathogen-induced TNF-α expression are well documented10-12. Mycobacteria, avian influenza and HIV-1 Tat protein are inducers of TNF-α through the MAP kinases13-15.
There are three MAP kinase subtypes including extracellular signal-regulated kinase-1/2 (ERK 1/2), p38 MAP kinase and c-Jun N-terminal kinase (JNK)16-20 known in humans. They transduce a variety of extracellular stimuli through a cascade of protein phosphorylations that lead to the activation of transcription factors such as NF-κB. The activation of NF-κB is crucial in production of cytokines including IL-6 and TNF-α13-15. The process occurs by the phosphorylation of I-κB at Ser32 and Ser36 via the I-κB kinase (IKK) signalosome complex followed by proteosomal degradation21 and consequent dissociation of I-κB and NF-κB subunits22. The activated NF-κB is then translocated from the cytoplasm to the nucleus, where it binds to κB binding sites in the promoter region of responsive genes, leading to the initiation of transcription of pro-inflammatory mediators. Because inappropriate activation of NF-κB is associated with a wide range of human diseases23, it has been considered as a plausible target for therapeutic intervention.
Currently, in the treatment for rheumatoid arthritis, psoriasis, psoriatic arthritis and ankylosing spondylitis, monoclonal TNF-α antibody plays an important role in the control of disease progression. Similarly, several randomized, double blind, placebo-controlled clinical trials had been performed in patients with Crohn's disease. The results of these clinical trials showed that the anti-TNF-α antibody (Infliximab) has beneficial effects to the patients24.
Additionally, recent studies showed that inflammatory responses including TNF-αproduction may play an important role in the pathogenesis of cardiovascular diseases (CVD). It has been suggested that TNF-α may destabilize the atherogenesis and atherosclerotic plaques leading to their rupture, resulting in myocardial infraction or stroke in CVD patients.
During microbial infection, macrophages are activated to produce cytokines to mediate immune response. Depending on the invading microbe and its biological properties, the host immune system utilizes different sets of cytokines to combat the invading pathogen locally and systemically.
A good example is mycobacterial infection, in which the proinflammatory cytokines TNF-α plays a critical role in host survival by propagating inflammation to contain the microbes by the formation of granuloma25. The protective role of TNF-α in controlling mycobacterial growth is exemplified by the resurgence of tuberculosis in patients receiving anti-TNF-αantibody therapy26.
Although the effects of proinflammatory cytokines are protective, their overproduction may have adverse effects to the host. In fact, uncontrolled induction of proinflammatory cytokine can lead to complications such as hypotension, organ failure and even death27, 28. Indeed, the overproduction of TNF-α in endotoxemia patients leads to serious deleterious symptoms. In chronic diseases such as rheumatoid arthritis, TNF-α overexpression is known to be the damaging factor and is associated with progressive joint destruction29.
In recent years, immunotherapeutics have been developed which aim at the neutralization of TNF-α and suppression of its undesirable proinflammatory effects. These include soluble TNF-α receptor (Enbrel) and anti-TNF-α antibody (Infliximab). Despite their novelty and efficacy in the arrest of disease progression, they are very expensive therapeutic regimens. Additionally, non-steroid anti-inflammatory drugs (NSAIDs) including aspirin, ibuprofen, and indomethacin are well-known in ameliorating acute and chronic pain associated with inflammatory diseases such as rheumatoid arthritis and inflammatory bowel disease. However, they are not effective in the treatment of advanced stages of rheumatoid arthritis and related autoimmune diseases. Also, these drugs are associated with severe adverse effects including gastrointestinal irritation, severe bleeding, and bone marrow suppression. Accordingly, therapeutics for treating inflammatory diseases and immune disorders are needed.